This invention relates to filter material for the removal of nitric oxide (NO) from smoke. More particularly, this invention relates to filter material containing colloidal silica, silica gel and a permanganate, which is effective in removing NO from tobacco smoke even after prolonged storage in the presence of moisture and the volatile components of tobacco.
Tobacco smoke produced by smoking articles such as cigarettes, pipes and cigars constitutes droplets and particles of materials suspended in a gas phase. The suspended droplets and particles constitute the total particulate matter (TPM) of the smoke. Both the TPM and the gas phase portions of tobacco smoke collectively produce the taste and aroma obtained from smoking a tobacco product.
The TPM can be removed in part from tobacco smoke by filtration devices attached to the smoking article which mechanically intercept and retain the suspended material. A porous rod of crimped fine-denier fibrous material is an example of such a filtration device.
It is preferred to remove some of the components, such as NO, from the gas phase of tobacco smoke. Granular filter materials for the removal of nitrogen oxides from cigarette smoke are disclosed in U.S. Pat. No. 3,434,479, which describes the use of various permanganates, including calcium, magnesium and sodium permanganates, deposited on various substrates, including bauxite, activated alumina, clay, kaolin, and silica gel.
Smoking articles such as cigarettes are packaged in a generally impermeable enclosure to preserve their freshness during the several months which may elapse before they reach the consumer. There is a high content of moisture and volatile organic compounds originating from the tobacco trapped inside the sealed package. Prior to being opened by the consumer, the package may be exposed to temperatures ranging from 30.degree. F. to 120.degree. F. After exposure to moisture, volatile organic compounds, temperature, and the passage of time, many materials which are initially active for smoke filtration lose much of their activity. The filter materials described in U.S. Pat. No. 3,434,479, when freshly prepared, remove a portion of the nitrogen oxides from cigarette smoke, but the activity of the materials diminishes so rapidly with aging and with exposure to moisture that they are impractical for commercial application.
Granular filter materials which remain effective after storage for the removal of nitrogen oxides are disclosed in U.S. Pat. No. 3,957,059, which describes activated alumina impregnated with NaMnO.sub.4 and a basic sodium compound, and which discloses that it is critical that the filter material contain less than 6% silica. Removal of 30% to 80% of the nitrogen oxides content of cigarette smoke after prolonged storage is disclosed.